There has been considerable interest in mirrorless spectrally narrowed laser-like emission from organic dyes and conjugated polymers. An early observation of such laserlike emission occurring in organic dyes was made by Mack (Appl. Phys. Lett. 15, 166 (1969)). Using short-pulse optical excitation of several polymethine cyanine dyes (cryptocyanine, 1′,1-diethyl-2,2′-dicarbocyanine iodide (DDI), and 3,3′-diethylthiatricarbocyanine iodide (DTTC)), Mack had obtained spectrally narrowed directional emission having typical linewidth of 13-18 nm with an energy conversion efficiency of up to about 3 percent. Following this work, several other groups reported experimental (U. Ganiel, A. Hardy, G. Neumann, and D. Treves, IEEE J. Quantum Elec. QE-11, 881 (1975), C. S. Wang, W. H. Cheng, C. J. Hwang, W. K. Burns, and R. P. Moeller, Appl. Phys. Lett. 41, 587 (1982)) and theoretical (L. Allen and G. 1. Peters, Phys. Lett. 31A, 95 (1970), L. W. Casperson and A. Yariv, IEEE J. Quantum Elec. QE-8, 80 (1972), L. W. Casperson, J. Appl. Phys. 48, 256 (1977)) investigation of spectral narrowing without feedback of the emitted radiation.
Glessner et al. (J. Appl. Phys. 62, 5 (1987)) reported amplified spontaneous emission (ASE) in the spectral range of 1.0-1.34 microns from an iodine filled cell utilizing a YAG laser pumped dye laser system as the excitation source, achieving about 1% conversion efficiency. However, later they achieved conversion efficiency of 8.3% at an elevated temperature and pressure conditions (U.S. Pat. No. 4,905,247 (1990)).
Significant advancement has been made in recent years in spectrally narrowed light emission using solid films of conjugated polymers. Frolov et al. reported mirrorless lasing in thin films of poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV), which they identified as superradiance (Jpn. J. Appl. Phys. 35, L1371 (1996)), or emission due to the formation of local cavities via scattering (OSA Annual Meeting, Paper ThG5, Santa Clara, September. 26-30 (1999)). Laser-like emission from such solid films usually can not continue beyond a maximum of 10,000 shots.
Hide et al. observed low gain narrowing threshold (1 μJ per pulse) in sub-micrometer thick films of semiconducting polymers based on waveguide structures (Science 273, 1833 (1996)). Superradiant emission in neat films of an alternating copolymer poly[dimethylsilylene-p-phenylenevinylene-(2,5-di-n-octyl-p-phenylene)-vinylene-p-phenylene] (Si-PPV) was observed by Brouwer et al. (Adv. Mater. 8, 935 (1996)). Spectral narrowing in optically pumped poly(p-phenylenevinylene) (PPV) films was reported by Denton et al. (Adv. Mater. 9, 547 (1997)).
Laser action was also observed in solutions of several conjugated materials placed in conventional resonator cavities (D. Moses, Appl. Phys. Lett. 60, 3215 (1992) and U.S. Pat. No. 5,237,582 (1993), H. Brouwer, V. V. Krasnikov, A. Hilberer, J. Wildeman, Appl. Phys. Lett. 66, 3404 (1995), W. Holzer, A. Penzkofer, S. Gong, A. Bleyer, and D. Bradley, Adv. Mater. 8, 974 (1996)).
Two-photon pumped up converted lasing has been demonstrated in dye doped polymer waveguides (A. Mukherjee, Appl. Phys. Lett. 62, 3423 (1993)), and solid matrices (He et al., IEEE J. Quantum Elec. 32, 749 (1996), Prasad et al., U.S. Pat. No. 5,912,257 (1999)). The present invention relates to laser-like emission using single-photon excitation.
In terms of applications, the major weaknesses of mirrorless laser-like emission have been the small conversion efficiency (<8%) and high pump energy thresholds (>10 μJ). Applications have not materialized because of these weaknesses. Clearly, appropriate materials that can lead to high conversion efficiencies (>15%) and lower thresholds (<5 microjoules) in mirrorless lasing are highly desired. Such a device would provide a low-cost source of short-pulse (picosecond) laser-like emission for various commercial and research related applications. These mirrorless devices would not require expensive optical elements, alignment accessories, and the extensive time needed for alignment and thus would substantially reduce the cost. The fact that dipolar structure may result in large conversion efficiencies and require low thresholds were not identified and reported in the literature so far. The present invention involves the invention that dipolar molecules which photoluminesce produce laser-like emission with exceptionally high conversion efficiency and at low pump-energy thresholds.